Homogeneous ruthenium—phosphine catalysts are useful for the reduction of 1,2-dioxygenated organic compounds, such as alkyl oxalates, glycolic acid, and glycolate esters to ethylene glycol. In particular, catalysts containing ruthenium in combination with tridentate phosphorus ligands such as, for example, 1,1,1-tris(diphenylphosphinomethyl)ethane (also known as “triphos”), have been used for the reduction of glycolic acid to ethylene glycol. These catalyst systems, however, are expensive and their economical use requires efficient recovery of the metal and ligand from the reaction products.
Optimizing recovery of the catalyst system is a complex problem involving several factors such as, for example, thermal degradation of the catalyst, the efficiency of the catalyst recovery process, and the impact of reaction by-product build-up in the reactor on both the reaction and downstream by-product separation steps.
The entire reactor effluent can be processed to recover the catalyst system from the reaction products and by-products. Alternatively, the reactor effluent can be processed to remove at least a part of the reaction products, and all or part of the concentrated reactor effluent can be processed to recover the catalyst system. The smallest catalyst recovery stream occurs when the reactor effluent is concentrated and most of the concentrated reactor effluent returns to the reactor with a minor amount of the concentrated reactor effluent, a purge stream, being processed to remove the reaction-by products and recycle the catalyst system to the reactor.
Extraction is an attractive means to recover the catalyst system from the reactor effluent in that extraction can readily be accomplished at low temperatures, minimizing the thermal degradation of the catalyst components. Feeding the reactor effluent directly to the extractor allows for minimum thermal degradation, but requires any water added to the extractor to be separated from the ethylene glycol product. This can economically limit the amount of water that can be added to the extractor and reduce catalyst system recovery efficiency. Concentrating the reactor effluent before feeding it to the extractor allows for significant flexibility in the amount of water added to the extractor, but may increase the thermal degradation of the catalyst components. Furthermore, a concentration step typically will not remove polyols which when recycled to the reactor can form unwanted diols such as 1,2-propanediol and 1,2-butanediol.
A method for the efficient recovery of ruthenium-1,1,1-tris(diaryl- or dialkylphosphinomethyl)alkane catalyst compositions from glycolic acid hydrogenation effluent to maximize recovery of the catalyst system at the lowest cost to the overall process, considering both reaction and separation steps, for producing ethylene glycol is desired.